Update from Gnulib

Update from Gnulib

This incorporates:
2019-12-06 nstrftime: better width support for %N, %z
2019-12-03 Avoid hassles caused by [[noreturn]] in C++
2019-12-02 Fix mistakes in --enable-threads=isoc fixes from 2019-12-01
* admin/merge-gnulib (AVOIDED_MODULES): Avoid pthread-h.
* lib/_Noreturn.h, lib/nstrftime.c, m4/gnulib-common.m4:
* m4/timer_time.m4:
Copy from Gnulib.
* lib/gnulib.mk.in: Regenerate.